Such an installation is known, for example, from GB-A 2 234 984 or DE 198 32 294 C1.
In order to reduce the pollutant emission during the combustion of the gasified fossil fuel or synthesis gas, a saturator, in which the synthesis gas is charged with steam during operation of the installation, can be connected into the gas line. For this purpose, the synthesis gas flows through the saturator in counter-flow to a flow of water, which is guided in a water circuit designated as a saturator circuit. In this arrangement, an input of heat from the water/steam circuit into the saturator circuit is provided for particularly high efficiency.
Due to the contact in the saturator with the heated flow of water guided in the saturation circuit, the gasified fuel is saturated with steam and experiences heating to a limited extent. For thermal technology reasons, and also for operational reasons, a further heating of the fuel by means of a heat exchanger can be necessary before its supply to the combustion chamber of the gas turbine.
In order to ensure particularly safe operation of the gas and steam turbine installation, it should be possible to stop a supply of the synthesis gas to the combustion chamber of the gas turbine at any time. For this purpose, a quick-action valve is usually connected into the gas line before the combustion chamber. If required, the quick-action valve blocks the gas line in a particularly short time so that no synthesis gas can reach the combustion chamber associated with the gas turbine.
Because of the relevant technical safety regulations, the fuel system usually includes a gas lock. A gas lock includes two valves, for example ball cocks, which have to be opened or closed for a through-flow of gas. An intermediate relief system or a pressure line is connected between these two valves. The intermediate relief system can be connected to a flare by which the surface gas can be burned off.
As an alternative to the intermediate relief system, it is possible to connect the pressure line, which ensures that no gas can flow in via the gas lock valves. The gas lock therefore separates, in a gas-tight manner, the fuel system into a first region; or the gasification system, upstream of the gas lock and into a second region; or the so-called gas turbine fuel system, downstream of the gas lock. Such a gas lock is known, for example, from Huth, M. et al.: “Combustion of synthesis gas in gas turbines”; BWK Brennstoff Wärme Kraft, DE, VDI Verlag GmbH, Düsseldorf, Vol. 50, No. 9, 09.01.1998, pp. 35-39, XP 000777542 ISSN: 0006-9612.
A gas and steam turbine installation with a gasification device can be operated both with the synthesis gas, which is for example generated from coal or residual oil, and with a second fuel, such as natural gas. In the case of a change from synthesis gas to the second fuel, or vice versa, it is necessary for safety technology reasons to flush the region between the gas lock and the combustion chamber, i.e. the gas turbine fuel system, with an inert medium such as nitrogen or steam. Because of the large volume of the region to be flushed, which in the appropriate case also includes the heat exchanger, flushing of this region is then required in both the forward and the rearward direction. This is done in order to achieve an almost complete displacement of the synthesis gas or of the second fuel and entering combustion gas, if necessary, out of the gas turbine fuel system.
Flushing with pure nitrogen, however, is found to be uneconomic because of the magnitude of the volume to be flushed. Flushing with steam, on the other hand, presupposes the provision of steam so that the flushing procedure is, as a rule, dependent on the operation of the waste-heat steam generator of the steam turbine installation. In this arrangement, furthermore, either cooling of the steam extracted from the waste-heat steam generator or protection of the components from particularly high steam temperatures is necessary. This is done so that the preparation for the flushing procedure is associated with a high level of complexity. In addition, complicated dewatering systems are necessary and the systems exposed to steam have to be protected from corrosion.
The flushing concept, which comprises forward and rearward flushing, leads to a complex control concept during the flushing operation and to particularly complicated control modes for the operation of the installation. Such a flushing concept also leads to relatively long installation run-up and run-down times.